U.S. patent number 4,416,759 [Application Number 06/325,588] was granted by the patent office on 1983-11-22 for sputter system incorporating an improved blocking shield for contouring the thickness of sputter coated layers.
This patent grant is currently assigned to Varian Associates, Inc.. Invention is credited to David J. Harra, Martin A. Hutchinson, Frederick T. Turner.
United States Patent |
4,416,759 |
Harra , et al. |
November 22, 1983 |
Sputter system incorporating an improved blocking shield for
contouring the thickness of sputter coated layers
Abstract
The arbitrary contouring of thickness of a sputter-deposited
film is made possible by a blocking means having a primary blocking
shield and an ancillary blocking shield. The primary blocking
shield intercepts atoms sputtered directly from the cathode target
by line-of-sight transport. The ancillary blocking shield extends
downwardly from the side of the primary blocking shield which is
placed closest to the substrate. The ancillary blocking shield
intercepts atoms sputtered from the cathode which by intervening
gas collisions have been redirected to travel underneath the
primary blocking shield. Precise tailoring of the thickness profile
of the coating on the substrate is thereby provided and the quality
of the composite film is maintained. The method for contouring the
thickness of sputter coated layers comprises sputter coating
portions of the film at successive stations. By employing the
improved blocking shield at selected stations, the film in the
aggregate may have the desired thickness contour.
Inventors: |
Harra; David J. (Santa Cruz,
CA), Turner; Frederick T. (Sunnyvale, CA), Hutchinson;
Martin A. (Santa Clara, CA) |
Assignee: |
Varian Associates, Inc. (Palo
Alto, CA)
|
Family
ID: |
23268510 |
Appl.
No.: |
06/325,588 |
Filed: |
November 27, 1981 |
Current U.S.
Class: |
204/298.11;
118/504; 118/505; 118/720; 118/721; 204/192.12; 204/192.13;
204/192.25 |
Current CPC
Class: |
H01J
37/34 (20130101); C23C 14/044 (20130101) |
Current International
Class: |
C23C
14/04 (20060101); H01J 37/32 (20060101); H01J
37/34 (20060101); C23C 015/00 () |
Field of
Search: |
;204/192R,192C,298
;118/720,721,504,505 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
738393 |
|
Jul 1966 |
|
CA |
|
486497 |
|
Jan 1976 |
|
SU |
|
Primary Examiner: Weisstuch; Aaron
Attorney, Agent or Firm: Cole; Stanley Z. Herbert; Leon
F.
Claims
What is claimed is:
1. A sputter system comprising an improved blocking shield for
contouring the thickness of a sputter-deposited layer, said sputter
system including a target cathode, a substrate mounting means and a
means for producing a plasma in said system, said improved blocking
shield comprising:
a primary blocking shield structure positioned in said sputter
system in between said target cathode and said substrate mounting
means, said primary blocking shield being positioned in spaced
adjacency to said substrate mounting means to shadow a portion of
said substrate mounting means from line-of sight exposure to said
target cathode, and
an ancillary blocking shield, said ancillary blocking shield
positioned in said sputter system to form an extension of said
primary blocking shield in the direction of said substrate mounting
means, said ancillary blocking shield being displaced inwardly from
the periphery of said blocking shield by a predetermined distance
and spaced a predetermined distance above said substrate mounting
means and serving to shield against the lateral movement of
sputtered atoms which are redirected inwardly of the bottom edge of
said primary blocking shield by gas scattering collisions.
2. A sputter system in accordance with claim 1 wherein said
substrate mounting means comprises means for mounting a generally
circular semiconductor wafer, said primary blocking shield
comprises a circular shield having a diameter less than the
diameter of said semiconductor wafer, and wherein said circular
primary blocking shield is positioned above and concentrically with
respect to said semiconductor wafer.
3. A sputter system in accordance with claim 2 wherein said
ancillary blocking shield comprises a circular ring having a
diameter less than the diameter of said primary blocking shield and
wherein said ancillary blocking shield is positioned concentrically
on the underside of said primary blocking shield.
4. A sputter system in accordance with claim 3 wherein said
ancillary blocking shield is positioned inwardly of said bottom
edge of said primary blocking shield a predetermined distance y,
between 0.1 inch and 0.6 inches.
5. A sputter system in accordance with claim 3 wherein said
ancillary blocking shield portion extends downwardly of said
primary blocking shield a predetermined distance x-z, between 0.1
inch and 0.5 inches.
6. A sputter system in accordance with claim 3 wherein the
predetermined distance, z, from the bottom of said ancillary
blocking shield to said substrate is between 0.03 inch to 0.1 inch.
Description
DESCRIPTION
This invention relates to an apparatus for contouring the thickness
of sputter-deposited layers and, more particularly, relates to an
improved blocking shield for arbitrarily contouring the thickness
of a sputter-deposited layer.
In the fabrication of semiconductor devices, it is increasingly
important to precisely control the uniformity of thickness of
coated layers. Uniformity in the thickness of coated layers can
increase yield, improve the performance of individual semiconductor
devices and may allow the devices across the wafer to more
consistently meet a particular specification. When semiconductor
wafers are coated by sputter coating, a material is sputtered off
of a target cathode in such a manner that it deposits on the
surface to be coated. See generally, R. W. Berry, et al., Thin Film
Technology, (1968) and L. J. Maissel, et al., Handbook of Thin Film
Technology, (1970). Such sputter coated films, for example, of
aluminum, are employed for the metallization of integrated
circuits. Heretofore, uneven films have either been tolerated or
attempts have been made to shape the sputtering cathode or to
arrange the geometry of the system in order to deposit as uniform a
film as possible.
One technique that has been utilized to enhance the uniformity of
sputtered films is the placement of blocking shields between the
target cathode and the substrate for at least a portion of the
sputtering cycle in order to intercept sputtered atoms which would
otherwise be applied to the substrate in areas of the film which
form thicker than other areas. In U.S. Pat. No. 3,856,654, of C. H.
George, "Apparatus for Feeding and Coating Masses of Workpieces in
a Controlled Atmosphere," a system is disclosed in which substrates
move on a circular track around a central cylindrical cathode. Each
substrate is exposed in succession to a particular segment of the
cathode. At most stations, the substrate is exposed to the cathode
without interference. However, at selected stations a fixed
blocking shield is used to shadow a portion of the substrate so
that sputtered atoms are only received by the unshadowed portions
of the substrate. By selecting the number of stations which have
blocking shields and by selecting the size and shape of the
blocking shields, the effect of non-uniformities which are
introduced by the system may be substantially reduced. More uniform
sputtered films may thereby be produced. In U.S. Pat. No.
3,904,503, A. M. Hanfmann, "Depositing Material on a Substrate
Using a Shield," a system is disclosed whereby planar shields of
various shapes and sizes are introduced between a cathode target
and a substrate. The inherent deposition characteristics of the
system are first observed, non-uniformities are noted and then the
shape, size and placement of the shield is determined. Shields of
variable shapes are also disclosed. The apparatus disclosed in
these two patents permits first order uniformity corrections to be
obtained. However, it has been found that since gas scattering is
also a transport mechanism that blockage of line-of-sight transport
does not provide complete shadowing and that the most optimum
uniformity is not necessarily obtained. Sputtered atoms which are
gas scattered will move underneath the shield and either add to the
film under the shield or deposit loosely on top of the film
deposited under the shield.
It is therefore an object of the present invention to provide
ancillary blockage to the sputtered atoms for enhancing the
uniformity correction obtained by the use of the primary blocking
shield placed inbetween a sputtering source and a substrate.
It is an additional object of the present invention to provide an
apparatus which prevents the transport of sputtered atoms by the
sputtering mechanism of gas collision onto portions of the
substrate shielded from line-of-sight deposition.
It is a still further object of the present invention to permit the
more precise contouring of thickness of a sputtered film by means
of an ancillary blocking shield used in conjunction with a primary
blocking shield.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention,
reference may be had to the accompanying drawings which are
incorporated herein by reference and in which:
FIG. 1 is a cross-sectional view of a sputtering station including
a magnetron source, primary blocking shield, substrate to be coated
and the ancillary blocking shield of the present invention;
FIG. 2 is a graph showing the contours of film thickness across the
wafer for layers formed in accordance with the method of the
present invention;
FIG. 3 is a perspective view of a multiple station
cassette-to-cassette coater for practicing the method and
incorporating the apparatus of the present invention; and
FIG. 4 is a graph showing the contours of film thickness in
accordance with the prior art (curve a) and with the present
invention (curves b and c).
SUMMARY OF THE INVENTION
The arbitrary contouring of thickness of a sputter-deposited film
is made possible by an ancillary blocking shield which is provided
for use in conjunction with a primary blocking shield. The
ancillary blocking shield extends downwardly from the side of the
primary blocking shield which is placed closest to the substrate.
The blocking shield intercepts atoms sputtered directly from the
cathode target generally by line-of-sight transport. The ancillary
blocking shield intercepts atoms sputtered from the cathode which
by intervening gas collisions have been directed underneath the
primary blocking shield. Precise tailoring of the thickness profile
of the coating on the substrate is thereby provided and the quality
of the composite film is maintained.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Sputter coating, particularly of metals, is an established process
in the semiconductor industry. Sputtering cathodes serve as targets
for receiving energetic gas ions and are composed of the material
to be deposited on the semiconductor wafer, e.g., aluminum,
aluminum-copper silicon, refractory metals and silicides or the
like. In order to obtain uniform coating on a substrate, the
targets may be shaped as disclosed for example in R. M. Rainey,
U.S. Pat. No. 4,100,055, "Target Profile for Sputtering Apparatus."
As seen in FIG. 3 of this patent, various profiles may be selected
for the target so that the distribution of the atoms sputtered from
the target over its life will prove to be as uniform as possible.
Notwithstanding the proper selection of a target profile, the
configuration of a given sputtering station, or the shape of a
substrate may introduce non-uniformities to the coating. For
certain applications, even if a uniform metal film can be deposited
it may be desirable or necessary to deliberately introduce a
non-uniformity so as to match, inversely, the non-uniformity of a
subsequent metal etching process step. Thus, it has been found
desirable to use shields which are positioned inbetween the target
cathode and the substrate. Such shields are generally shown, for
example, in U.S. Pat. Nos. 3,856,654 and 3,904,503, discussed
previously. These shields are typically planar structures which are
interposed between the target cathode and the substrate. Thus,
atoms sputtered from the cathode will not reach those areas of the
substrate which are normally coated more thickly than desired.
Generally, the shields intercept those atoms which would travel
from a point on the target by line-of-sight transport to these
certain areas on the substrate. Incidentally, they intercept a
portion of these atoms which experience a gas scattering collision
on their way to the substrate. In a sputtering chamber, gas
scattering collisions occur because a sputtering atmosphere on the
order of about 1.times.10.sup.-3 torr to about 5.times.10.sup.-2
torr is maintained, as shown schematically by gas cloud 20 in FIG.
1. There is thus a finite probability that a sputtered atom will
collide with an atom or molecule of the sputtering gas, e.g.,
argon, during its travel to the substrate. However, not all of
these sputtered atoms that have been gas scattered are intercepted
by the prior art primary blocking shield.
It has been found that some of the sputtered atoms experience
scattering collisions with the sputtering gas adjacent the edge of
the blocking plate and develop a component to their direction of
travel which is parallel to the surface of the substrate. This is
illustrated in FIG. 1 by the sputtered atom having trajectory 14
which experiences a gas scattering collision at point 15. As a
result of this collision the sputtered atom is redirected to travel
along new trajectory 16 underneath the blocking shield and would be
deposited on the supposedly shaded portion of the semiconductor
wafer. These spurious deposits are found to integrate into the film
as it forms and thereby degrade the quality of the film or are
found to not adhere at all and must be cleaned from the surface of
the film. Such spurious gas scattered deposits may produce films
whose microscopic structure has properties such as grain size and
reflectivity which may be markedly different from the properties
desired. As seen in FIG. 1, the sputtered atom with trajectory 13
has travelled from sputter cathode 37 to the center of the primary
blocking shield 11, whereas the sputtered atom with trajectories
14, 16 has travelled to the center of the substrate, the very place
desired to be shaded by primary blocking shield 11. This detracts
from the ability to arbitrarily contour the thickness of the
sputtered layer, permits the spurious deposits to form in the
regions which should otherwise be blocked and may degrade the
quality of the sputtered layer.
In theory, the blocking means 10 could be placed immediately
adjacent but displaced from wafer slightly so as to maintain a
noncontact condition, and the edge effect and gas scattering
mechanism would not be especially troublesome. However, it is
necessary to position the blocking means 10 sufficiently above the
substrate 17 so that primary blocking shield 11 is displaced a
distance x above the surface of substrate 17 in order to avoid a
step function discontinuity in the sputtered film, i.e., in order
to gradually blend the edge of the layer deposited on the unblocked
portion with the edge of the layer deposited on the blocked
portion. The distance x is great enough so there is no contact with
the wafer, but not so great that the shield now becomes an
obstruction and intercepts material excessively or so that gas
scattered deposits may penetrate to the center of the wafer.
The present invention involves the addition of an ancillary
blocking shield 12 to primary blocking shield 11 to intercept the
spurious gas scattered atoms described above. Thus, the sputtered
atom with trajectory 16 will now be intercepted.
The ancillary blocking shield 12 of the present invention permits
the blocking shield 11 to be positioned an appropriate distance x
above semiconductor wafer 17 and yet prevents sputtered atoms from
travelling by gas scattering in a horizontal direction underneath
the primary blocking shield.
When the sputtering system is set up to coat a semiconductor wafer,
then blocking shield 11 will be circular and the ancillary blocking
shield comprises a ring 12 which extends downwardly from primary
blocking shield 11. Ring 12 may be attached to shield 11 or may be
formed integrally therewith. Ring 12 may be orthogonal to shield 11
or may be shaped as desired. Ring 12 is displaced a distance y
inwardly from the periphery of primary blocking shield 11. In a
sputtering system for coating semiconductors, ring 12 is typically
placed on primary blocking shield 11 in concentric fashion. Values
for these variable distances are chosen in order to tailor the
profile of the thickness of the sputtered coating as shown in FIG.
2.
A typical system for utilizing the improved blocking shield is
shown in FIG. 1. A cathode having circular symmetry such as conical
magnetron source 30 is placed in opposition to a semiconductor
wafer holding apparatus 23. The operation of such a conical
magnetron source is described in L. T. Lamont, Jr., "A Magnetically
Enhanced Sputter Source for Semiconductor Metallizations,"
Proceedings, 8th Intl. Vacuum Congress, France (1980). Briefly, a
glow discharge plasma is generated above cathode 37 by maintaining
a significant potential difference between anode 34 and cathode 37
in the presence of a sputtering atmosphere. The cathode is actively
cooled by introducing a coolant via conduits 31 to cavity 32. Wafer
holding apparatus 23 is of the type used in coating stations in the
multiple station coater of FIG. 3. Spring clips 18 hold
semiconductor wafer 17 on a recessed lip defined by peripheral
ledge 22 in base 24. Base 24 sits within an opening defined by
plate 21. In the embodiment of FIG. 1, wires 19, attached to plate
21, hold blocking shield 11 of blocking means 10 over semiconductor
wafer 17.
In FIG. 2 curve a shows the contour of film thickness when a
centrally disposed primary blocking shield has been used to form a
final portion of the film. In this case, the shape of the shield
and the deposition time have been selected to fill in a shallow
coating around the circumference and to build up the
circumferential layer above the rest of the wafer. The buildup is
about 10% of the total film thickness over the center. It can be
seen that the thickness is greatest around the circumference and
drops off gradually as the unblocked and blocked portions blend
together. Although the curves do not show it since no ancillary
blocking shield was used, there is a likelihood that the film
quality has degraded due to the presence of spurious gas scattered
atoms. Curve b shows sputtering with the ancillary blocking shield
used along with the primary blocking shield. Portions of the curve
b are similarly shaped to curve a but displaced due to a slightly
larger diameter for the primary blocking shield for curve b. The
cutoff of the lower portion of curve b at c represents a shadowing
of the substrate by the ancillary blocking shield. Here, although
the curves do not show it, a high quality is maintained for the
sputtered film since the spurious contributions of gas scattered
atoms are curtailed. It has been found that the distance y that the
ancillary blocking shield is displaced from the edge of the primary
blocking shield, as shown in FIG. 1, will determine this cutoff
point. The height x of the primary blocking shield above the
substrate 17 will determine the proper amount of gas scattered
material for the blend. The height z of the bottom of ancillary
blocking shield 12 above the surface will determine the safety
factor for non-contact with the wafer.
A novel sputtering sequence has been adopted for utilizing the
improved blocking shield of the present invention. This sequence is
premised upon having a positive-hold in-line sputter deposition
system such as the 3180 Cassette Coating System available from
Varian Associates, inc., Palo Alto, Calif. Prior art sputter
deposition systems typically involve batch loading and batch
coating of semiconductor wafers. Alternately, they used a
continuous feed mechanism as exemplified by the Triad System of MRC
Corporation, Orangeburg, N.Y. In a positive-hold in-line system,
each wafer is presented to a specific work station and held there
for a finite period until a specific function is performed. As
shown in FIG. 3 and as illustrated in the product brochures
entitled "3180 Cassette Coating System-the Cassette-to-Cassette
Coater," available from the Palo Alto Vacuum Division of Varian
Associates, Inc., 611 Hansen Way, Palo Alto, Calif. 94303,
semiconductor wafers are carried in cassettes 45, 46, 47 along
track 56. For processing, a semiconductor wafer is loaded from a
cassette 46 by an automatic wafer loading mechanism 48 through a
vacuum load lock onto one of a plurality of positive-hold stations
on revolving plate 50. The semiconductor wafer undergoes processing
at each successive position as plate 50 is rotated about its
central axis 51 by belt drive 55. Preheating, deposition, sputter
etching, cooling or other similar steps, may be accomplished at the
successive stations. With such an in-line system having discrete
stations, it is possible to carry out sputter coating in successive
steps.
The method of the present invention comprises combining in
succession at least two sputter deposition steps. At least one of
the steps is carried out in a sputter deposition station which
utilizes a blocking shield such as the improved blocking shield of
the present invention. The contours of film thickness of coatings
may be tailored by combining the individual profiles of each
successive step, e.g., the thickness profile of an unblocked
sputter source and the thickness profile of a shielded source.
Alternately, shields of strikingly different shapes with different
thickness profiles may be used in succession. By varying the
residence time at each station, great flexibility is obtained in
tailoring the cumulative thickness profiles. For example, station
52 could serve to preheat the wafer. Then, after the wafer is
indexed to station 53, a standard sputtering operation could be
carried out to apply a metal layer having a thickness profile
characteristic of the system, e.g., the profile of curve a in FIG.
4. Finally, after indexing to station 54 a further layer of metal
is applied by a sputtering system in which a blocking shield is
employed. The shape of the shield is selected to compensate for the
nonuniformity introduced at station 53. Since semiconductor wafers
are round, a circular shield of the type shown in FIG. 1 may be
employed. The diameter of the shield and the height above the wafer
are chosen so that the additive annular metal layer has a shape
akin to that of curve c of FIG. 4 and so that the cumulative metal
layer has a thickness profile which is nearly a straight line,
i.e., the thickness of the film is highly uniform, as shown in
curve b of FIG. 4. It should be appreciated that there are several
variations of the method of operation. These include:
(1) unshielded deposition followed by a shielded deposition with
time proportions such that a uniform film is produced.
(2) unshielded deposition followed by a shielded deposition with
time proportions such that a deliberately non-uniform film is
produced, e.g., a film is produced which is thicker around an
annular ring at the periphery of the wafer.
(3) Shielded deposition at a first station followed by a shielded
deposition at a second station with the two shields and the time
proportions being selected to produce an arbitrary contour required
for a particular application.
In all of these method sequences, the improved blocking shield of
the present invention or a generalized blocking shield may be used.
The order in which the steps are to be performed is not critical;
however, for best results, it is preferable to deposit most of the
metal first without a blocking shield and then to tailor the
cumulative contour by depositing metal using a blocking shield.
* * * * *